Seoul , South Korea -LRB- CNN -RRB- -- The South Korean Navy fired warning shots to ward off North Korean fishing boats that were spotted south of the maritime border between the two countries , the South Korean Joint Chiefs of Staff said Friday .

The North Korean vessels crossed back over the border , known as the Northern Limit Line , following the warning shots , according to the Joint Chiefs of Staff .

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The South Korean Navy issued two warning messages to the North Korean boats before firing a few dozen shots , the Joint Chiefs of Staff said , adding that the alert level for the navy had been raised .

The were no immediate reports on the incident Friday by the North 's official Korean Central News Agency .

The two Koreas , which remain technically at war following their conflict in the early 1950s , have periodically clashed at sea over the years , with some waters still in dispute . The land border between the two nations remains heavily militarized .

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Fatal naval skirmishes took place near the inter-Korean maritime border in the Yellow Sea in 1999 , 2002 and 2009 .

And in 2010 , Seoul accused Pyongyang of sinking one of its naval vessels near disputed waters off North Korea , killing 46 South Korean sailors . North Korea has repeatedly denied responsibility for causing the sinking .

South Korea is a key ally of the United States , with tens of thousands of U.S. troops stationed in the country .

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The North Korean fishing boats retreated after the warning , the South says

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The South Korean Navy issued warning messages before firing

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Fatal naval clashes have taken place periodically between the two Koreas